Experimental Study On Cryogenic Environment Drilling Of Carbon Fiber Reinforced Plastics

Carbon fiber composites have been widely used in aerospace,wind turbine blades,sports,automobiles etc.due to their high strength,high modulus,small thermal expansion coefficient,good fatigue and corrosion resistance.Drilling is one of the main processing forms of carbon fiber composites.However,due to the anisotropy and high hardness of carbon fiber composites,the cutting heat is difficultly dissipated during drilling processes,making the materials susceptible to damage,such as outlet tearing,burrs,pits and other defects that will not only greatly affect the material application and safety performance,but also restrict the application of carbon fiber composites in high-end equipments.Therefore,how to achieve efficient and low-damage drilling of carbon fiber composite materials has always been an important topic of the research.Based on the low temperature properties of carbon fiber composites,this paper proposed an experimental study on the drilling processes with the liquid nitrogen（LN₂）cooling medium.The main works were as follows:The mechanical and physical properties of carbon fiber composites at different temperatures were concluded.The low-temperature-cutting mechanism of carbon fiber composites and hole formation processes of drilling were analyzed,which provided a theoretical basis for studying the inhibition mechanism of carbon fiber composites on the drilling damage under the low temperature condition.The acoustic emission monitoring tests of low-temperature-drilling processes of carbon fiber composites were carried out.Using the single-factor analysis,the processing quality variations during the liquid nitrogen cooling and normal temperature dry drilling processes were compared as well as the effective values of acoustic emission signals.Based on the hole processing qualities of drilling,the influence of liquid nitrogen cooling and process parameters on the hole outlet qualities were analyzed.Comparing with the dry drilling processes,the optimal processing parameter range without tear defects during liquid nitrogen cooling processes was obtained.And the processing mechanism of the hole wall was analyzed by observing and analyzing the surface morphology.The morphology and surface roughness of the hole wall under normal temperature dry and liquid nitrogen low temperature cooling conditions were compared.The experimental study on tool wear during drilling processes under liquid nitrogen cooling condition was carried out.The relationship between the number of holes and tool wear,the influence of tool wear on hole’s outlet qualities and the vibration signals of the hole’s outlet were analyzed.The maximum number of holes when the tool can meet the processing requirements at the optimum feed speed was obtained,and the effectiveness of using vibration signals to monitor tool wear was verified.From the results of liquid nitrogen low-temperature-drilling acoustic emission monitoring tests and tool wear tests,drilling under the liquid nitrogen cooling condition could effectively reduce the instantaneous temperature of the drilling area,slow down the tool wear,improve the tool life,and restrain the outlet tearing of the holes.Also the burr damages during drilling processes were greatly reduced.What’s more,under the low feed,there was no obvious pits phenomenon on the hole walls,and the qualities of the hole walls were significantly improved.As a result,high-quality,low-damage processing processes of carbon fiber composites were achieved.